The present invention relates to a variable gain amplifier and, more particularly, to a variable gain amplifier for use in a video camera system or the like, which is capable of varying a gain in response to levels of a gain control signal.
Variable gain amplifiers are used in many electronic circuits and, particularly, are used for level controlling of picture signals of a video camera system. As a conventional variable gain amplifier having a gain controllable range of among 1/n times.about.1 time.about.n times, there is a circuit as illustrated in FIG. 1.
An amplifier circuit 1.sub.A has a gain of nA times (A being a real number excepting "0", and n being a real number larger than 1: the same hereinafter). An attenuator circuit 5 attenuates an output from the amplifier circuit 1.sub.A within a range of from 0.about.1 times in response to a gain control signal V.sub.GC applied thereto and outputs the attenuated output to an output terminal T.sub.O. Further, the attenuator circuit 5 attenuates the output from the amplifier circuit 1.sub.A to 1/n times at a central value within the controllable range of the gain control signal V.sub.GC.
Now, it is assumed that an attenuation ratio of the attenuator circuit 5 with respect to an input is set to "1" when a level of the gain control signal V.sub.GC is the maximum, is set to 1/n at a central voltage value, and is set to "0" at the minimum value. Then, when the level of the gain control signal V.sub.GC is the minimum value, a level v.sub.o of an output signal V.sub.O takes the following form, with a level of an input signal V.sub.I being v.sub.i : EQU v.sub.o =v.sub.i .times.nA.times.0=0 (1)
When the level of the gain control signal V.sub.GC is the central value, the level v.sub.o takes the following form: EQU v.sub.o =v.sub.i .times.nA.times.1/n=Av.sub.i ( 2)
When the level of the gain control signal V.sub.GC is the maximum value, the level v.sub.o takes the following form: EQU v.sub.o =v.sub.i .times.nA.times.1=nAv.sub.i ( 3)
A gain variable characteristic in this case is shown in FIG. 2. That is, this variable gain amplifier has a gain controllable range of 0 times.about.A times.about.nA times.
A specific circuit example of this conventional variable gain amplifier is illustrated in FIG. 3.
In this circuit, transistors Q11 and Q12, constant current sources I1 and I2, and resistors R11.about.R13 and R17 cooperate with one another to form the amplifier circuit 1.sub.A having the gain of nA, and transistors Q13 and Q14 and resistors R14.about.R17 cooperate with one another to form the attenuator circuit 5 whose attenuation amount varies in response to the level changes in the gain control signal V.sub.GC. An emitter area ratio between the transistors Q13 and Q14 is set to (1/1n):(1/n).
Here, in order to realize a variable gain amplifier whose gain controllable range is among 0 times.about.1 time.about.2 times on the assumption that A=1 and n=2, a resistance ratio between the resistors R11 and R17 should be set to 1:2 and an emitter area ratio between the transistors Q13 and Q14 should be set to 1:1.
Furthermore, as described previously, in order to realize a variable gain amplifier whose gain controllable range is among A/n times.about.A times.about.nA times, the minimum level of the gain control signal V.sub.GC should not be "0", but the gain control signal V.sub.GC should be such a level that the attenuation amount of the attenuator circuit 5 will be 1/n.sup.2.
The conventional variable gain amplifier described above has a following disadvantage. That is, the level controlling range of the gain control signal V.sub.GC is not of a symmetrical characteristic with respect to the central value (V.sub.GCCEN) of the gain control signal V.sub.GC if an attempt is made on setting the gain controllable range among A/n times.about.A times.about.nA times. This is because such conventional variable gain amplifier is so arranged that the minimum value of the gain becomes "0" when the gain control signal V.sub.GC is rendered to the minimum value.
Further, there is another disadvantage. That is, for setting the minimum gain of the gain variable amplifier to A/n times, it requires that the level of the gain control signal V.sub.GC be critically set due to such influence as variations in characteristics of the attenuator circuit 5, thereby making it difficult to set the minimum gain.